docs: def key(self, identifier=None, security_level=None):
          """
          Tries to find an encryption key, first using the ``identifier`` and
          if that fails or isn't provided using the ``security_level``.
          Returns ``None`` if nothing matches.
          """
          if identifier:
              try:
                  return self._encryption_keys[identifier]
              except KeyError:
                  pass
          if security_level:
              for key in self._encryption_keys.values():
                  if key.level == security_level:
                      return key